全新java初学者实践教程10(java SE5.0版)
w}c1zpa jdk5的集合类
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Z3<>Z\6D 上次课我们学过了数组,知道它只是一组数(或是对象),但是有些自己的特性。在[font="Times]java里还有一类东西与数组类似,也是有着特性的一组数[font="Times](或是对象[font="Times]),叫做集合类。
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<4@RrB 我们上节课讲到了,数组的长度在创建时已经确定了,但是有时候我们事先根本不知道长度是多少啊,比如我们做电子商务网站时,有个购物车程序。你总不能用数组规定,人家只能买[font="Times]5样东西吧。你就是把长度定为[font="Times]10000也不行,万一遇上个特别有钱的呢!呵呵,这只是开玩笑的。我们会使用集合类解决这个问题。
MUh) [font="Times] 集合类是放在[font="Times]java.util.*;这个包里。集合类存放的都是对象的引用,而非对象本身,为了说起来方便些,我们称集合中的对象就是指集合中对象的引用([font="Times]reference)。引用的概念大家不会忘了吧,在前边我们讲
数据类型时讲的。
mk2T };o6|e:2E [font="Times] 集合类型主要有[font="Times]3种:[font="Times]set(集)、[font="Times]list(列表)、[font="Times]map(映射[font="Times])和[font="Times]Queue(队列)。[font="Times]//队列为[font="Times]jdk5中的加上的[font="Times]
G"T)+!6t pk%I98! Jy [font="Times](1) Set
hZ Gr/5f 集([font="Times]set)是最简单的一种集合,它的对象不按特定方式排序,只是简单的把对象加入集合中,就像往口袋里放东西。对集中成员的访问和操作是通过集中对象的引用进行的,所以集中不能有重复对象。我们知道数学上的集合也是[font="Times]Set这个,集合里面一定是没有重复的元素的。
wj!p6D;;S *91iFeKj= ([font="Times]2)[font="Times]List
{a.{x+!5I- 列表([font="Times]List)的主要特征是其对象以线性方式存储,没有特定顺序,只有一个开头和一个结尾,当然,它与根本没有顺序的[font="Times]Set是不同的。它是链表嘛,一条链肯定有顺序这个顺序就不一定了。
8>RGmue p%EU,:I6 *v)JX _ [font="Times] ([font="Times]3)[font="Times]Map
Q^$IlzG7i [font="Times] 映射([font="Times]Map),这个在[font="Times]java里不是地图的意思,其实地图也是映射哈。它里面的东西是键-值对([font="Times]key-value)出现的,键值对是什么呢?举个例子,比如我们查字典,用部首查字法。目录那个字就是键,这个字的解释就是值。键和值成对出现。这样说可以理解吧。这也是很常用的数据结构哦。
:WIbjI= Q:&,8h[ M7-piRnd4 [font="Times] ([font="Times]4)[font="Times]Queue
XAR~d6iZ [font="Times] 在[font="Times]jdk5.0以前,通常的实现方式是使用[font="Times]java.util.List集合来模仿[font="Times]Queue。[font="Times]Queue的概念通过把对象添加(称为[font="Times]enqueuing的操作)到[font="Times]List的尾部(即[font="Times]Queue的后部)并通过从[font="Times]List的头部(即[font="Times]Queue的前部)提取对象而从[font="Times] List中移除(称为[font="Times]dequeuing的操作)来模拟。你需要执行先进先出的动作时可以直接使用[font="Times]Queue接口就可以了。
)HNbWGu [font="Times] 这[font="Times]4个东西,有时候功能还不太完善,需要有些子类继承它的特性。[font="Times]Set的子接口有[font="Times]TreeSet,SortedSet,[font="Times]List的有[font="Times]ArrayList等,[font="Times]Map里有[font="Times]HashMap,HashTable等,[font="Times]Queue里面有[font="Times]BlockingQueue等。我们来看看例子吧:
r483"k(7 _v{,vLH 060<wjX6 实践:[font="Times] Set举例
l~!Tnp\M [font="Times] import java.util.*;
~
nNsq(4 [font="Times]public class SetExample {
_6Wz1.]n [font="Times] public static void main(String[] args) {
"(PJh\S>S [font="Times] Set set = new HashSet(); //HashSet是[font="Times]Set的子接口
3Q*K+(`{ [font="Times] set.add("one");
[wG?&l$.KB [font="Times] set.add("second");
tQ_;UQlX [font="Times] set.add("3rd");
g6o-/A!Q3 [font="Times] set.add(new Integer(4));
*M\Qt_[ [font="Times] set.add(new Float( 5.0F ));
U>7"BpC [font="Times] set.add("second");
hSSF] [font="Times] set.add(new Integer(4));
0kS[`a(}J [font="Times] System.out.println(set);
M;OY+|uA [font="Times] }}
Vh$~]>t:f :BKY#uH~ [font="Times]List举例:
+8Yt91 [font="Times] import java.util.*;
:P# [font="Times]public class ListExample {
-BfZ P5 [font="Times] public static void main(String[] args) {
3Wxl7"!x m [font="Times] List list = new ArrayList();
b)9bYkd [font="Times] list.add("one");
KT~J@];Fb [font="Times] list.add("second");
%Ez%pT0TQ# [font="Times] list.add("3rd");
O|m-Uz"+ [font="Times] list.add(new Integer(4));
?6+GE_VZ [font="Times] list.add(new Float( 5.0F ));
6[,*2a8 [font="Times] list.add("second");
"(5A5> [font="Times] list.add(new Integer(4));
xfCq;?MupW [font="Times] System.out.println(list);
{LYA?w^GT [font="Times] }}
pj;cL]L 7GY[l3arxv /1:`?% ,2 [font="Times]Map举例
hPF9y@lh [font="Times]import java.util.Map;
ugcWFB5| [font="Times]import java.util.HashMap;
A1e| Y [font="Times]import java.util.Iterator;
km,}7^?F0r [font="Times]import java.io.FileReader;
mV^+`GWvo [font="Times]
I$xfCu [font="Times]public class MapExample {
cG1-.,r [font="Times] public static void main(String[] args) throws java.io.FileNotFoundException {
oNY;z-QK [font="Times] Map word_count_map = new HashMap();
[_*% [font="Times] FileReader reader = new FileReader(args[0]);
YqX/7b+ [font="Times] Iterator words = new WordStreamIterator(reader);
VFz(U)._ [font="Times]
2#~5[PtP^ [font="Times] while ( words.hasNext() ) {
KGdL1~ [font="Times] String word = (String) words.next();
@;2,TY>Di [font="Times] String word_lowercase = word.toLowerCase();
8`XpcK-0 [font="Times] Integer frequency = (Integer)word_count_map.get(word_lowercase);
zm4Okg)w@ [font="Times]if ( frequency == null ) {
li;Np5P [font="Times] frequency = new Integer(1);
+Wh0Of [font="Times] } else {
vS%o>"P [font="Times] int value = frequency.intValue();
(.4mX
t [font="Times] frequency = new Integer(value + 1);}
z1wy@1o' [font="Times] word_count_map.put(word_lowercase, frequency);
3$[!BPLFO [font="Times] }
:"7V,UP
@ [font="Times] System.out.println(word_count_map);
]n1@!qa48 [font="Times] }}
.9{Sr[P SjV;&
1Z/ "& 'h\ [font="Times]Queue举例:
cdVh_"[ [font="Times]import java.io.IOException;
'hfQ4EN [font="Times]import java.io.PrintStream;
]f#ZU{A'mt [font="Times]import java.util.LinkedList;
-8;U1 ^# [font="Times]import java.util.Queue;
"f/lm 2< [font="Times]
/{
Lo0 [font="Times]public class QueueTester {
uoR_/vol8 [font="Times] public Queue q; //发现了一个奇怪的语法,这个尖括号是泛型声明
?.~E:8 [font="Times] public QueueTester() {q = new LinkedList();}
s9svuFb [font="Times]public void testFIFO(PrintStream out) throws IOException {
~K]5`(KV [font="Times] q.add("First");
|S[Gg [font="Times] q.add("Second");
LPX@oh a [font="Times] q.add("Third");
v?BX 4FO [font="Times] Object o;
hZf0q 2 [font="Times] while ((o = q.poll()) != null) {
Jj4!O3\I [font="Times] out.println(o);}}
+#7e?B [font="Times] public static void main(String[] args) {
W- 5Z"m1I [font="Times] QueueTester tester = new QueueTester();
.)~IoIW= [font="Times] try { tester.testFIFO(System.out);
Cfqgu;m [font="Times] } catch (IOException e) {
ts/rV#s~ [font="Times] e.printStackTrace(); } }}
#8iRWm0*6 :5IbOpVM Mu$9#[/ 上述例子和一些相关例子(共6个例子)打包下载[font="Times]
4<g,L;pUU 总结:
kaf4GME] 刚才我们看了上述例子了,对集合类有了一个初步的认识,它们跟数组的区别不只是长度问题,在集合类里面放进去的类型可以是任意的。不像数组,数组里面的类型是一样的。这样的话,对于集合类来说即使好事,也是坏事。因为你不考虑类型可以随意的放,但是你放进去什么就不知道了不容易找。
T#%r\f,l0 还有啊,什么叫泛型声明啊?我们下次课就告诉你。这可是[font="Times]jdk5的酷炫之处哦。